Unify behaviour of nqp::substr on all backends

[usev6]

This is the current behaviour of nqp::substr on the different backends with regard to negative values for $position and $length:

my $s := 'foobar'

Code	Result on MoarVM
nqp::substr($s, 4, -1)	'ar'
nqp::substr($s, 4, -2)	dies: Substring length (-2) cannot be negative
nqp::substr($s, -4)	'obar'
nqp::substr($s, -4, 2)	'ob'
nqp::substr($s, -4, -1)	'obar'
nqp::substr($s, -4, -2)	dies: Substring length (-2) cannot be negative
nqp::substr($s, -7, 1)	''
nqp::substr($s, -8, 1)	dies: Substring end (-1) cannot be less than 0
nqp::substr($s, -8)	'foobar'

Code	Result on JVM
nqp::substr($s, 4, -1)	dies: StringIndexOutOfBoundsException: String index out of range: -1
nqp::substr($s, 4, -2)	dies: StringIndexOutOfBoundsException: String index out of range: -2
nqp::substr($s, -4)	'obar'
nqp::substr($s, -4, 2)	dies: StringIndexOutOfBoundsException: String index out of range: -4
nqp::substr($s, -4, -1)	dies: StringIndexOutOfBoundsException: String index out of range: -4
nqp::substr($s, -4, -2)	dies: StringIndexOutOfBoundsException: String index out of range: -4
nqp::substr($s, -7, 1)	dies: StringIndexOutOfBoundsException: String index out of range: -7
nqp::substr($s, -8, 1)	dies: StringIndexOutOfBoundsException: String index out of range: -8
nqp::substr($s, -8)	dies: StringIndexOutOfBoundsException: String index out of range: -2

Code	Result on JS
nqp::substr($s, 4, -1)	''
nqp::substr($s, 4, -2)	''
nqp::substr($s, -4)	'obar'
nqp::substr($s, -4, 2)	'ob'
nqp::substr($s, -4, -1)	''
nqp::substr($s, -4, -2)	''
nqp::substr($s, -7, 1)	'f'
nqp::substr($s, -8, 1)	'f'
nqp::substr($s, -8)	'foobar'

IMHO the behaviour should be identical. This begs the question: What behaviour do we want? For Rakudo we don't allow negative values, but allow '*-2' to index relative to the end.

For the records: This is the output from Perl 5's substr.

Code	Result from Perl 5
substr($s, 4, -1)	'a'
substr($s, 4, -2)	''
substr($s, -4)	'obar'
substr($s, -4, 2)	'ob'
substr($s, -4, -1)	'oba'
substr($s, -4, -2)	'ob'
substr($s, -7, 1)	''
substr($s, -8, 1)	''
substr($s, -8)	'foobar'

If the semantics of nqp::substr are changed for MoarVM, all uses within NQP and Rakudo need to be reviewed. There might be some ecosystem fallout, too.

Another for the record: This is the current implementation on MoarVM: https://github.com/MoarVM/MoarVM/blob/423a9cc7d9/src/strings/ops.c#L706